Peel and dice the celeriac, parsnip and apple.
Thinly chop the onion and garlic.
Heat the oil in a pot, then gently sauté the onion for 5 minutes. Add the garlic, then stir for a minute.
Add the chopped vegetables bay leaf and the stock then bring to the boil and turn down the heat.
Put a lid on and gently simmer for 10 minutes, then add the apple and cinnamon . Cook for another 5 minutes or so until tender.
Meanwhile, cut the onion into thin rings. Heat the oil in a frying pan and cook the onion over low heat for 8 minutes. Add the vinegar, cinnamon and honey or maple syrup, then caramelise for a couple of minutes.
When the soup is ready, remove the bay leaf, then blitz until you get a smooth soup, using a stick blender.
Serve with the caramelised onion and some seeds.